Polyetherimides can be prepared by the self-condensation of hydroxyaryl phthalimide salts, as disclosed in U.S. Pat. No. 4,297,474 (which is incorporated by reference into this description). The polymers have the general formula ##STR3## wherein R is a trivalent C.sub.(6-13) aromatic organic radical, R' is a divalent C.sub.(6-30) aromatic organic radical, and X is --O-- or --S--. The polymers have alternating imide and ether (or thioether) linkages between aromatic radicals. Similar polyetherimide polymers are prepared by the reaction of alkali metal dihydric phenol and an organic bis(fluorophthalimide) in the presence of a dipolar aprotic solvent, as disclosed in U.S. Pat. No. 3,847,869 (which also is incorporated by reference into this description). Polysulfoneimides of the same general type are prepared by the reaction of an aromatic bis(sulfoneanhydride) with an organic diamine, as disclosed in U.S. Pat. No. 4,107,147 (which also is incorporated by reference into this description). While these etherimide and sulfoneimide polymers are suitable for films, coatings, etc., their solvent-resistance and other physical properties in composite form can be improved by adding crosslinking end cap functionalities to the polymer backbones, thereby making the composites (cured from the oligomers) better suited for high performance applications, such as aerospace needs.